US military commanders are planning to pull back their troops from Iraq's towns and cities and redeploy them in four giant bases in a strategy they say is a prelude to eventual withdrawal.

The plan, details of which emerged at the weekend, also foresees a transfer to Iraqi command of more than 100 bases that have been occupied by US-led multinational forces since the invasion of Iraq in March 2003.

However, the decision to in vest in the bases, which will require the construction of more permanent structures such as blast-proof barracks and offices, is seen by some as a sign that the US expects to keep a permanent presence in Iraq.

Politicians opposed to a long-term US presence on Iraqi soil questioned the plan.

"They appear to settling in a for the long run, and that will only give fuel for the terrorists," said a spokesman for the mainstream Sunni Iraqi Islamic party.

A senior US official in Baghdad said yesterday: "It has always been a main plank of our exit strategy to withdraw from the urban areas as and when Iraqi forces are trained up and able to take the strain. It is much better for all concerned that Iraqis police themselves."

Under the plan, for which the official said there was no "hard-and-fast" deadline, US troops would gradually concentrate inside four heavily fortified air bases, from where they would provide "logistical support and quick reaction capability where necessary to Iraqis". The bases would be situated in the north, south, west and centre of the country.

He said the pace of the "troop consolidation" would be dictated by the level of the insurgency and the progress of Iraq's fledgling security structures.

A report in yesterday's Washington Post said the new bases would be constructed around existing airfields to ensure supply lines and troop mobility. It named the four probable locations as: Tallil in the south; Al Asad in the west; Balad in the centre and either Irbil or Qayyarah in the north.

US officers told the paper that the bases would have a more permanent character to them, with more robust buildings and structures than can be seen at most existing bases in Iraq. The new buildings would be constructed to withstand direct mortar fire.

A source at the Iraqi defence ministry said: "We expect these facilities will ultimately be to the benefit of the domestic forces, to be handed over when the US leaves."

· Three Romanian journalists kidnapped in Iraq were freed yesterday after two months in captivity.